Polymeric compositions, which incorporate lubricating oils that bleed to the surface of the composition to perform a lubricating function, have been disclosed in the prior art. In U.S. Pat. No. 4,486,319, Jamison, the inventor of the composition claimed below, showed that the properties of certain plastics able to exude oil can be modified by incorporating solid materials to conduct away frictional heat. Further, in U.S. Pat. No. 4,915,856, he described a lubricating composition formed of polymers, oils, and both solid and liquid agents that were added to modify the lubricating performance of the composition.
Other plastic compositions that have improved tribological properties include thermoplastic resins in which various solid and liquid lubricants are incorporated into their structure, such as polyethylene, polyamide (nylon), and polyoxymethylene (acetal). Solid lubricants that have been incorporated into nylon and acetal include powders of graphite, molybdenum disulfide, and polytetrafluorethylene (PTFE). In addition, PTFE fibers have been randomly dispersed in certain plastic resins. In U.S. Pat. No. 4,945,126, Crosby and Theberge blended polyethylene and PTFE to form an additive for use in a nonspecific resin system, to reduce friction and wear. Examples of molded and extruded solid forms of acetals containing PTFE are DELRIN AF.TM. (DuPont Corporation) and LUBRICOMP KL4010.TM., 4020.TM., and 4030.TM. (LNP Engineering Plastics, Inc.). Nylon compositions that contain molybdenum disulfide are NYLATRON GS.TM. and NYLATRON GSM.TM. (Polymer Corporation).
Solid lubricants that are incorporated into resin systems for use in forming structural components, such as bearings and gears, generally exist as dispersed separate phases. In contrast, resin systems that incorporate liquid lubricants into their structure to achieve improved tribological properties either create dispersed separate phases of the liquid and solid or totally dissolve the liquid in the solid resin phase. Compositions in which the liquid remains totally dissolved in the resin do not provide optimum lubrication. In fact, much better lubrication performance is observed when the liquid is slowly rejected from the interior of the composition to continually form a thin film on the surface of objects made from the resin. To achieve this result, it is best to select a liquid lubricant and a resin system that have limited mutual solubility, and to process the composition so that the liquid is dispersed within the resin and bleeds to the surface at a limited rate. Bearing compositions are disclosed by Lankamp in U.S. Pat. No. 4,448,700; these compositions comprise specific neopentylpolyol esters dispersed in polypropylene. Takishi et al. disclose polyethylene formulations in U.S. Pat. No. 5,079,287 that incorporate both liquid and solid lubricants. Other examples of related lubricant compositions are discussed in U.S. Pat. No. 3,135,564 (using vinyl chloride); U.S. Pat. No. 4,146,487 (using polymethyl pentene); U.S. Pat. Nos. 3,541,011; 3,547,819; and 3,729,415 (in which Davis et al. teach formulation of lubricant dispensing materials from polyethylenes).
Silicone oils have been infused into a variety of polymers, such as nylon, to improve their tribological properties. Examples include PLASLUBE.TM. NY-1/SI/5 (Ako Engineering Plastics, Inc.), and STANYL.TM. TW341 (DSM Engineering Plastics).
Representative internally lubricated acetals are FULTON 441.TM. (LNP Engineering Plastics), ACETRON NS.TM. (Polymer Corporation), CELCON LW90.TM. (Hoechst-Celanese Corporation), and DELRIN 500CL.TM. (DuPont Corporation). In each of these prior art oil infused compositions, the oils are totally dissolved within the solid polymer, and thus, do not provide optimum lubrication. In the commercial composition OILES 80.TM. (Oiles America Corporation), a liquid lubricant is dispersed in an acetal resin, but the lubricant does not diffuse throughout the polymer and is only released as wear of the polymer exposes pockets of the oil.
Infusion of silicone oils and other lubricating oils into acetals in amounts sufficient to optimize the tribological properties of the composition have been found to impair the ability to process these compositions into useful objects by extrusion or injection molding. This problem arises due to the inherent immiscibility of acetals and most lubricating oils. Specifically, when pellets of highly oil-infused acetals are loaded into an injection molding machine or extruder, the heat generated by the machine causes oil to bleed to the surface of the pellets. The oil then transfers to the screws and barrels of the machine and lubricates their surfaces so that the frictional forces developed within the machine are inadequate to create the pressures within the dies required to form the material. This problem, which is recognized in the prior art, is referred to as a "transport problem."
In U.S. Pat. No. 4,041,002, Aboshi et al. teach a method to improve the transport properties of oil-infused acetals. They note that by mixing with the acetal an amount of an ethylene vinyl acetate sufficient to absorb the oil, the resulting composition can be pelletized for subsequent molding or extrusion. The types of oils that they employ are restricted to those that are insoluble in acetal, but soluble in ethylene vinyl acetate. However, to achieve the concentration of oil required for acceptable tribological performance, a relatively large concentration of ethylene vinyl acetate must be added, which degrades the mechanical performance, and processing of the resulting composition.